The invention relates to a screw spindle pump having a housing in which a drive spindle and at least one running spindle having profile assemblies which engage inside each other are received, the free end of the drive spindle and the end portion of the running spindle being supported on a plate which is arranged transversely in the housing.
Screw spindle pumps of this type are used, for example, as fuel pumps. DE 41 23 384 A1 discloses a fuel conveying unit having a housing in which an electric motor is arranged. The shaft of the electric motor is connected to a drive spindle in a rotationally secure manner, but the transmission of force of the motor to the drive spindle can also be introduced by means of a coupling element (see DE 43 08 755 A1). As a result of the rotation of the motor or the drive spindle, the running spindle is caused to rotate, As a result of the profile assemblies which are also referred to as conveying profiles and which are provided on the drive spindle and the running spindle(s), there are formed together with the housing displacement chambers through which a conveyed fluid is conveyed axially from the intake side to the pressure side. In the screw spindle pump known from DE 41 23 384 A1, the free end of the drive spindle and the end portion of the running spindle are axially supported on a plate which is referred to therein as a bearing face. The fluid conveyed flows past the bearing face at both sides.
Although this fuel pump has been found to be advantageous for many decades, it has been found that, under specific conditions, depending on component tolerances and pressure relationships, an undesirable movement of the spindles may occur. This movement is undefined or irregular; it primarily involves crossing of the spindles. This crossing results in the longitudinal axes of the at least two spindles not being arranged in a precisely parallel manner but instead forming an angle with each other, Therefore, the longitudinal axes of the at least two spindles can also be displaced or crossed with respect to a notional plane in which both longitudinal axes are ideally located, one spindle protruding from one side of the notional plane and the other spindle from the opposing side of the notional plane. This spindle movement which may be observed leads to disadvantageous effects. On the one hand, the profile gaps increase, primarily at the intake side of the screw spindle pump, whereby the leakage of the pump increases. This occurs in particular at elevated medium temperatures, which leads to impaired heat conveying behaviour. The increase of the profile gaps also leads, on the other hand, to a reduction of the degree of efficiency. In addition, the undefined movement of the spindles brings about increased suction pulsation which is also undesirable. The rolling line formed between the two or more spindles is influenced in a negative manner by friction effects, whereby the drive torque which has to be applied by the electric motor is increased. Furthermore, the undesirable wobble movement of the spindles also leads to increased wear.
FIG. 1 shows a plate I of a conventional screw spindle pump, The plate 1 is in the form of a feather key and has a planar upper side 2, The two arrows 3, 4 symbolically represent the longitudinal axes of a drive spindle and a running spindle. In FIG. 1, it can be seen that the two spindles carry out a wobble movement at the upper side 2, that is to say, the ends of the spindles “travel” which results in an oblique position which can be seen particularly well in FIGS. 2 and 3. In FIG. 2, it can be seen that the spindles starting from a parallel starting position at the intake side of the screw spindle pump, at the upper side 2 of the plate 1, have moved apart. FIG. 3 is a view of FIG. 2 rotated through 90° and it can be seen here that the spindles have also moved in a non-uniform manner relative to a notional plane in which ideally parallel spindle axes are located so that the spindle 4 is located in front of this notional plane and the other spindle 3 is located behind this notional plane.